The present invention relates to a system for encoding and/or decoding a plurality of samples of a speech signal in a lump or block unit and more particularly, to a block digital processing system for nonuniformly encoded digital words for use in various communications systems such as a PCM telephone exchange system or the like.
The transmission of analog signals such as a speech signal and a video signal can be efficiently achieved in digital form through band compression utilizing the inherent redundancy of these signals. This redundancy is attributed to the fact that an amplitude distribution of a speech signal is nonuniform and that there is a correlation between the speech signals. A band compression technique using the redundancy based on the former reason is known as a quantizing technique or a variable length encoding technique. Also, a band compression technique resorting to the latter reason is known as a DPCM (differential pulse code modulation) or delta modulation technique.
A block digital processing system for nonuniformly encoded digital words performs band compression using the redundancy caused by the above-mentioned two reasons, in which the band compression is achieved by converting samples of a speech signal into nonuniformly encoded digital words each of which is represented by a sign bit to indicate a polarity of the sample, segment bits to indicate a range to which said amplitude value of the sample belongs, and mantissa bits to indicate at what location in said range said amplitude value exists, reencoding said digital words through the uniform quantization in the block unit of a predetermined number of samples, and transmitting a block code to indicate the step size of the quantization levels (abbreviated hereunder as the step size) and said reencoded codes.
For details of such a system, reference is made to U.S. Pat. No. 3,945,002. In the system of FIG. 1 of that Patent, a transmitter digital processor comprises shift registers (111.about.118) for storing and delaying nonuniformly encoded input digital words corresponding to samples of a speech signal by an integral multiple of the speech sampling period (one block period), a maximum detector (121.about.126) for detecting a sample value having the maximum amplitude in one block of said nonuniformly encoded digital words, an encoder (133, 149, 152.about.157 and 160) for reencoding output digital words given from said shift registers through the uniform quantization according to the step size determined by said maximum value, and a multiplexer (190) for combining the output codes fed from said encoder and a block code to indicate information relating to said step size and transmitting the combined codes through a transmission line, while a receiver digital processor comprises a demultiplexer (290) for separating the output codes of the encoder and the block code given from the output codes of said transmitter digital processor, and decoders (233, 255, 271, 272, 260) for decoding said output codes of the encoder into said nonuniformly encoded input digital words on the basis of the step size indicated by said block code.
However, in this conventional system, the step sizes in the encoder and decoder are preset at 6 dB (decibels) intervals, and as a result, this system allows the valleys of 6 dB to arise in the S/N (signal-to-noise) characteristics representing the relationship between the S/N ratio and the input signal amplitude (dB) relative to a full load when a sinusoidal wave signal is given as the input signal, deteriorating the S/N ratio greatly and resulting in degradation of the performance of the system. On the other hand, setting the step size more finely by adopting the prior art construction for the purpose of eliminating this shortcoming results in the system becoming bulky and costly.